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Subject:	Re: protecting program memory
To:	djgpp AT delorie DOT com
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From:	Gordon DOT Schumacher AT seagate DOT com
Date:	Fri, 23 Mar 2007 16:37:27 -0600
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tim DOT nicholson AT skyforcekent DOT com wrote on 23 Mar 2007 04:16:50
-0700:

# Is there an easy way to protect the memory used to store the
# program op codes from a rogue memory write from within the
# application? I have a large (100,000+ line) DJGPP
# application which sometimes crashes with SIGILL - It would
# seem the program is cannibalizing itself! In order
# to find how this is happening, I would like to protect the
# entire block of memory that contains the application code
# so that an exception occurs at the point the corruption occurs
# rather than the point that the corrupted code is executed.
#
# I guest I need to make the memory block read only, but I am
# not sure how to do that.

Not without DPMI 1.0, you can't.

But here's an alternative: checksum the executable space.

You can get a pointer to the beginning of code and its size
by applying some mild abuse of GCC and the linker map:

   extern char*               _text  asm(".text");
   extern char*               _etext asm("etext");
   static char*               __my_progstart   = NULL;
   static size_t              __my_progsize    = 0;
   __my_progstart = (char*) &_text;
   __my_progsize = (&_etext - &_text) - sizeof(void*);

There are any number of things that you can do now, armed
with that data!  (Look into instrumentation with GCC...)

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